1. Field of Invention
This invention is concerned with techniques for producing a plurality of optical patterns and more particularly with a technique for rapidly changing the optical patterns.
2. Description of the Prior Art
There is a need to image multiple optical patterns on an object for the purpose of ranging to the object. The ranging is useful, for example, in robotics applications where it is desired to know the distance from one object, such as a robot arm, to an object to be manipulated thereby. Typically the patterns in such an imaging system are bar codes with varying spatial frequencies from one pattern to another. One prior art solution to create such bar code patterns is the use of a film projector and an endless film strip with successive film frames having different optical patterns. The problem with such a system is that the film projector, having many mechanical parts, quickly wears out and the film, typically mylar film, breaks.
A second solution is the use of a light modulator such as a liquid crystal array. This has the distinct advantage over the film projector-film arrangement in that there are no moving parts. However, liquid crystals change optical patterns only relatively slowly and the types of liquid crystal arrays suitable for creating optical images are very expensive, of the order of fifty thousand dollars.